The horizontal is seconds, the vertical is ounces. The steep slope is a shot with the brew water at 190˚. You can see it's flowing much faster. The two on the right are shots with water at about 202˚. The graph is useful, I think, but is very tedious to produce. For instance, it's interesting that the flow rates don't curve gently upward, but rather have distinct moments of increased flow. On the 202˚ shots, at about 18-20 seconds you can see that the flow rate suddenly doubles. Why is this? (The bumps in the lines are from the fact that the espresso is flowing in big, goopy drops.)
I'm trying to find all the variables that go into producing a consistent shot. The big ones are temperature of the brew water and the amount of grounds in the puck. All 3 shots used 0.75 ounces of grounds. The only other variable, I believe, is the pressure used to tamp the puck. There is a device for this. It utilizes a mechanism similar to a torque wrench.